1,965 research outputs found
Actively stressed marginal networks
We study the effects of motor-generated stresses in disordered three
dimensional fiber networks using a combination of a mean-field, effective
medium theory, scaling analysis and a computational model. We find that motor
activity controls the elasticity in an anomalous fashion close to the point of
marginal stability by coupling to critical network fluctuations. We also show
that motor stresses can stabilize initially floppy networks, extending the
range of critical behavior to a broad regime of network connectivities below
the marginal point. Away from this regime, or at high stress, motors give rise
to a linear increase in stiffness with stress. Finally, we demonstrate that our
results are captured by a simple, constitutive scaling relation highlighting
the important role of non-affine strain fluctuations as a susceptibility to
motor stress.Comment: 8 pages, 4 figure
Stress relaxation in F-actin solutions by severing
Networks of filamentous actin (F-actin) are important for the mechanics of
most animal cells. These cytoskeletal networks are highly dynamic, with a
variety of actin-associated proteins that control cross-linking, polymerization
and force generation in the cytoskeleton. Inspired by recent rheological
experiments on reconstituted solutions of dynamic actin filaments, we report a
theoretical model that describes stress relaxation behavior of these solutions
in the presence of severing proteins. We show that depending on the kinetic
rates of assembly, disassembly, and severing, one can observe both
length-dependent and length-independent relaxation behavior
Critical behaviour in the nonlinear elastic response of hydrogels
In this paper we study the elastic response of synthetic hydrogels to an
applied shear stress. The hydrogels studied here have previously been shown to
mimic the behaviour of biopolymer networks when they are sufficiently far above
the gel point. We show that near the gel point they exhibit an elastic response
that is consistent with the predicted critical behaviour of networks near or
below the isostatic point of marginal stability. This point separates rigid and
floppy states, distinguished by the presence or absence of finite linear
elastic moduli. Recent theoretical work has also focused on the response of
such networks to finite or large deformations, both near and below the
isostatic point. Despite this interest, experimental evidence for the existence
of criticality in such networks has been lacking. Using computer simulations,
we identify critical signatures in the mechanical response of sub-isostatic
networks as a function of applied shear stress. We also present experimental
evidence consistent with these predictions. Furthermore, our results show the
existence of two distinct critical regimes, one of which arises from the
nonlinear stretch response of semi-flexible polymers.
Magnetic anisotropy of vicinal (001) fcc Co films: role of crystal splitting and structure relaxation in step-decoration effect
The uniaxial in-plane magnetic anisotropy (UIP-MA) constant is calculated for
a single step on the (001) surface of fcc Co() films. The calculations are
done for both an undecorated step and the step decorated with one or more, up
to 7, Cu wires. Our objective is to explain the mechanisms by which the
decoration decreases the UIP-MA constant, which is the effect observed
experimentally for ultrathin Co films deposited on vicinal (001) Cu surfaces
and can lead to reorientation of magnetization within the film plane.
Theoretical calculations performed with a realistic tight-binding model show
that the step decoration changes the UIP-MA constant significantly only if the
splitting between the on-site energies of various -orbitals is included for
atoms located near the step edge. The local relaxation of atomic structure
around the step is also shown to have a significant effect on the shift of the
UIP-MA constant. The influence of these two relevant factors is analyzed
further by examining individual contributions to the UIP-MA constant from atoms
around the step. The magnitude of the obtained UIP-MA shift agrees well with
experimental data. It is also found that an additional shift due to possible
charge transfer between Cu and Co atoms is very small.Comment: 12 pages,9 figures, RevTeX, submitted to Physical Review B version 3:
additions to content version 2: minor correction
Length-Controlled Elasticity in 3D Fiber Networks
We present a model for disordered 3D fiber networks to study their linear and
nonlinear elasticity over a wide range of network densities and fiber lengths.
In contrast to previous 2D models, these 3D networks with binary cross-links
are under-constrained with respect to fiber stretching elasticity, suggesting
that bending may dominate their response. We find that such networks exhibit a
fiber length-controlled bending regime and a crossover to a stretch-dominated
regime for lengths beyond a characteristic scale that depends on the fiber's
elastic properties. Finally, by extending the model to the nonlinear regime, we
show that these networks become intrinsically nonlinear with a vanishing linear
response regime in the limit of floppy or long filaments.Comment: 4 pages, 4 figure
Multiple light scattering in nematic liquid crystals
We present a rigorous treatment of the diffusion approximation for multiple
light scattering in anisotropic random media, and apply it to director
fluctuations in a nematic liquid crystal. For a typical nematic material, 5CB,
we give numerical values of the diffusion constants and .
We also calculate the temporal autocorrelation function measured in Diffusing
Wave Spectroscopy.Comment: 5 pages RevTeX, 1 postscript figure, to be published in Phys. Rev. E
(Rapid Communication
Bi-defects of Nematic Surfactant Bilayers
We consider the effects of the coupling between the orientational order of
the two monolayers in flat nematic bilayers. We show that the presence of a
topological defect on one bilayer generates a nontrivial orientational texture
on both monolayers. Therefore, one cannot consider isolated defects on one
monolayer, but rather associated pairs of defects on either monolayer, which we
call bi-defects. Bi-defects generally produce walls, such that the textures of
the two monolayers are identical outside the walls, and different in their
interior. We suggest some experimental conditions in which these structures
could be observed.Comment: RevTeX, 4 pages, 3 figure
One-Bead Microrheology with Rotating Particles
We lay the theoretical basis for one-bead microrheology with rotating
particles, i.e, a method where colloids are used to probe the mechanical
properties of viscoelastic media. Based on a two-fluid model, we calculate the
compliance and discuss it for two cases. We first assume that the elastic and
fluid component exhibit both stick boundary conditions at the particle surface.
Then, the compliance fulfills a generalized Stokes law with a complex shear
modulus whose validity is only limited by inertial effects, in contrast to
translational motion. Secondly, we find that the validity of the Stokes regime
is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let
Barrier and internal wave contributions to the quantum probability density and flux in light heavy-ion elastic scattering
We investigate the properties of the optical model wave function for light
heavy-ion systems where absorption is incomplete, such as Ca
and O around 30 MeV incident energy. Strong focusing effects
are predicted to occur well inside the nucleus, where the probability density
can reach values much higher than that of the incident wave. This focusing is
shown to be correlated with the presence at back angles of a strong enhancement
in the elastic cross section, the so-called ALAS (anomalous large angle
scattering) phenomenon; this is substantiated by calculations of the quantum
probability flux and of classical trajectories. To clarify this mechanism, we
decompose the scattering wave function and the associated probability flux into
their barrier and internal wave contributions within a fully quantal
calculation. Finally, a calculation of the divergence of the quantum flux shows
that when absorption is incomplete, the focal region gives a sizeable
contribution to nonelastic processes.Comment: 16 pages, 15 figures. RevTeX file. To appear in Phys. Rev. C. The
figures are only available via anonynous FTP on
ftp://umhsp02.umh.ac.be/pub/ftp_pnt/figscat
n-atic Order and Continuous Shape Changes of Deformable Surfaces of Genus Zero
We consider in mean-field theory the continuous development below a
second-order phase transition of -atic tangent plane order on a deformable
surface of genus zero with order parameter . Tangent plane order expels Gaussian curvature. In addition, the total
vorticity of orientational order on a surface of genus zero is two. Thus, the
ordered phase of an -atic on such a surface will have vortices of
strength , zeros in its order parameter, and a nonspherical
equilibrium shape. Our calculations are based on a phenomenological model with
a gauge-like coupling between and curvature, and our analysis follows
closely the Abrikosov treatment of a type II superconductor just below
.Comment: REVTEX, 12 page
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